The present disclosure relates to apparatus for sealing and severing a subsea pipeline adapted to convey fluids, e.g., hydrocarbons, and systems and methods using such apparatus.
The demand for safe, environmentally friendly decommissioning of subsea oil and gas pipelines no longer in service is increasing. Significant amounts of operating capital are expended to remove such pipelines, many times within operating oil and gas production fields. A common current practice is to pull the decommissioned pipelines to the surface to be cut on deck of a surface vessel. In some cases, a pipeline to be removed cannot be lifted to the ocean surface in one piece because of congestion on the seafloor of pipelines, electrical cables, communication cables and the like. In some cases, a pipeline to be removed cannot be safely pulled to the surface in one piece because of corrosion, a particular problem in deep water. For these and other reasons, decommissioned subsea pipelines are typically cut into sections ranging from a few hundred feet to a few thousand feet in length before being lifted to the surface for removal and disposal. The cut sections are typically not sealed at the ends, and they may be left on the seafloor for various periods of time in which they can be subject to dislocation in the event of a strong currents in the area.
Subsea pipelines to be decommissioned are typically flushed and pigged prior to cutting the pipelines to remove hydrocarbons and thereby reduce the risk of hydrocarbon discharge during decommissioning and abandonment. High points and dead legs in the pipeline can trap hydrocarbons and preclude their being effectively cleaned out during this process. Physical constraints can also preclude the ability to pig some pipelines. Pipelines that have been conveying hydrocarbons with high paraffin content over time can contain build up of paraffin or wax deposits which may not be sufficiently cleaned by flushing and/or pigging. One current practice is for divers to manually cut the pipelines into sections with containment hoods covering the pipelines to contain any potential hydrocarbon release. This can be a time-consuming, expensive and dangerous process. Many pipelines are then sealed and abandoned in place and buried under the seafloor.
What is needed is a method of decommissioning subsea pipelines with reduced risk of hydrocarbon release to the environment, improved process safety and increased productivity.